This invention relates to a dry planographic printing plate whereby printing can be effected without dampening water. More specifically it relates to a dry planographic printing plate suitable for an in-house or in-plant printing.
There has long been applied widely and practically a planographic printing method using dampening water in which image area and non-image area are distinguished from each other by a physicochemical difference based on the intervention of water and the reproduction of image is effected. This method, however, has various drawbacks caused by using water, and, in order to solve the drawbacks, there have been proposed various dry planographic printing methods requiring no dampening water.
The dry planographic printing plate used in these proposed methods operates by distinguishing image area from non-image area by coating the non-image area on the printing plate with an ink repellent substance, e.g. an organosilicone polymer or an organofluorine compound. In a method for selectively forming an ink repellent area in the non-image area, a printing plate composed of a base substrate, a photosensitive layer and an ink repellent substance layer is image-wise exposed to the light and the exposed area or unexposed area is removed with a developer.
In another method an image is formed with an ink receptive toner by the electrophotography on a printing plate composed of a base substrate and a layer of an ink repellent substance. The latter method is simple in forming an image, and so is suitable for an in-house or in-plant printing, but this method has a serious problem that the durability, printing endurance of the printing plate, is much inferior to a printing plate using dampening water, because the toner image does not firmly adhere to the surface of an ink repellant substance.
In Japanese Patent Laid Open Publication No. 29305/1977 it is proposed that an organopolysiloxane having a reactive organic group can be effectively used as the ink repellant layer. According to this proposed method, background contamination in non-image area and printing endurance are remarkably improved. Nevertheless there still remains a problem in the balance between background contamination and printing endurance so this method is not yet considered satisfactory for practical application. Moreover, an organosiloxane having a reactive organic group is expensive and the use thereof is uneconomical. On the other hand, Japanese Patent Laid Open Publication No. 1803/1975 proposes that an organopolysiloxane which constitutes the ink repellent layer be modified for example with polystyrene and that a heterogeneous polymer be used comprising an ABA type tri- or multi-block polymer. Further, in Japanese Patent Laid Open Publication No. 66008/1976 there is proposed the use of a block polymer consisting of a siloxane block and a thermoplastic organic block as an ink repellent elastomer layer. Printing plate surfaces made according to these proposed methods exhibit a fairly improved adhesion to toner as compared with a printing plate surface consisting of an unmodified organopolysiloxane alone, but are still unsatisfactory for practical application in view of the balance between background contamination and printing endurance (in general, there is an inverse correlation such that if background contamination is remedied the printing endurance lowers, and if the printing endurance is improved the background contamination becomes worse). Besides, since for the production of those modified polymers a peculiar polymerization method should be employed, such substances for printing plate surface are not satisfactory also from the economic point of view.
In Japanese Patent Laid Open Publication No. 76104/1977 there is proposed the use of a solid fluoro-compound copolymer as an ink repellent substance. As a result of using a fluoro-compound copolymer, the adhesion to toner is improved, but the printing plate surface becomes less repellent against ink, so that it becomes necessary to give consideration to the printing machine and the printing paper and therefore, it is not easy to obtain a printed matter free from background contamination. In addition, a fluoro-compound is costly to manufacture and the use thereof is uneconomical.
Organopolysiloxanes and organofluorine compounds exhibit a repellent property against ink but at the same time do not allow an image-forming substance, e.g. toner, to adhere securely thereto, thus forming a printing plate surface of a low printing endurance and unsatisfactory for practical application. If an organopolysiloxane is further modified to enhance its bonding force to an image-forming substance, it becomes less repellent against ink, and background contamination easily occurs. A dry planographic printing ink, in general, is different from a planographic printing ink using a dampening water, and it is impossible to allow the latter to serve also as the former. For effectively using a printing plate which is liable to cause background contamination, it is necessary to give some consideration to ink as one measure, but it is difficult to design an ink maintaining its characteristics such as tack, flow and stability which is convenient for use. A dry planographic printing plate which can maintain the adhesion of toner strongly without lowering the ink repellency on the plate surface, if available, would be extremely convenient from the standpoint of both production and use of ink.
It is an object of this invention to solve the above-mentioned problems associated with the prior art.
It is another object of this invention to provide a dry planographic printing plate for direct image forming superior in both ink repellency and toner adhesion.
Other objects and advantages of this invention will become apparent from the following description.